This invention relates to a method for the shutdown and restarting of a combined plant including, in combination, a gas turbine and a steam turbine. More particularly, the invention is concerned with a method of reducing the restarting time of the combined plant.
A combined plant including a gas turbine and a steam turbine is suitable to use as a medium size heat power plant which is started and shutdown everyday, because the gas turbine is capable of being started quickly and the steam turbine has a superb starting and shutdown characteristic under the low-pressure and low-temperature steam conditions (approximately, 50 to 80 Kg/cm.sup.2 g and 450.degree. to 500.degree. C.).
In shortening the time required for starting the combined plant, the end can be attained by reducing the starting time for the steam turbine to shorten the overall starting time of the combined plant, since it has already been possible to start the gas turbine in about ten minutes. The end of shortening the starting time for the steam turbine can be attained by shutting down the steam turbine while maintaining the steam turbine metal at a high temperature level at the time of shutdown and by restarting the steam turbine while the steam turbine metal is maintained at a high temperature level, because the variation in temperature of turbine metal from the time the steam turbine is started to the time the steam turbine operates at rated load can be reduced and thus thermal stress can be reduced if the temperature of turbine metal can be kept at a high level as aforesaid. Thus, the reduction in the restarting time of the combined plant can be achieved.
However, in the combined plant including in combination a gas turbine and a steam turbine, the temperature of the waste gas from the gas turbine has a characteristic such that the lower the load imposed on the gas turbine, the lower the temperature of the waste gas becomes. And the temperature of steam generated in a waste heat recovery boiler by utilizing the waste gas of the gas turbine becomes lower as the load is reduced. Thus, the combined plant has had the disadvantage that the temperature of the main steam supplied to the steam turbine also drops when the plant is shut down, so that the temperature of the steam turbine metal drops when the steam turbine is shut down.